High-voltage vacuum tube, notably an X-ray tube, comprising a metal sleeve

ABSTRACT

An X-ray tube comprises a cathode structure surrounded by a metal sleeve and supported on a metal base secured to the sleeve by means of an insulator which is provided with a circular groove for supporting a metal screen arranged between the base and the sleeve to prevent electric discharge from the sharp edge of the base.

[451 Oct.7,1975

United States Patent [1 1 Peter NOTABLY AN X-RAY TUBE, COMPRISING Primary Examiner-R. V. Rolinec A METAL SLEEVE Assistant ExaminerDarwin R. Hostetter Attorney, Agent, or Firm-Frank R. Trifari; George B. [75] Inventor: Berka HIGH-VOLTAGE VACUUM TUBE,

Diethard Peter, Hamburg, Germany Assignee: U.S. Philips Corporation, New

York, NY.

221 Filed: Jan. 4, 1974 211 App]. No.: 430,790

ABSTRACT An X-ray tube comprises a cathode structure sur- [30] Foreign Application Priority Data Jan. 8, 1973 rounded by a metal sleeve and supported on a metal base secured to the sleeve by means of an insulator which is provided with a circular groove for supporting a metal screen arranged between the base and the sleeve to prevent electric discharge from the sharp edge of the base.

[51] Int. [58] Field of [56] References Cited UNITED STATES PATENTS 3 Claims, 1 Drawing Figure 2,332,426 10/1943 Atlee....................................313/59 S t v 34 US. Patent Oct. 7,1975

HIGH-VOLTAGE VACUUM TUBE, NOTABLY AN X-RAY TUBE, COMPRISING A METAL SLEEVE The invention relates to a high-voltage vacuum tube, notably an X-ray tube, comprising an anode and a cathode which are enclosed by a metal sleeve, the cathode being mounted on a metal base which is connected to the metal sleeve by way of an insulator.

X-ray tubes of this kind are known (for example, from US. Pat. No. 2,332,426). The part of the metal base which is connected to the insulator has cmparatively sharp edges which can readily give rise to flashover between the metal base and the metal sleeve either via the insulator or through the vacuum. For this reason, use has been made of screening sleeves which completely cover the edge of the metal base, which carry the same potential as the base during operation, and whose extremity has been rounded so as to form a thick edge. As a result, only small field strengths can arise on the ends thereof and the risk of flash-over is substantially reduced. The thickened edge requires substantial space, so that the X-ray tube cannot have a compact construction. However, if use is made of a screening sleeve whose thickened end has a comparatively small radius of curvature, energy-rich discharges between the screening sleeve and the insulator or the metal sleeve cannot be effectively prevented. So as to eliminate this drawback, it is known to cover the thickened end of the screening sleeve with an insulating material. However, because only comparatively thin vacuum-resistant cover layers can be made (for example, of glass), the risk exists that the glass layer is charged and subsequently breaks down. Glass dust is then produced which causes the high-voltage strength of the X-ray tube to be substantially reduced.

Finally, it is also known to construct the insulator so as to have a wave-like shape, thus preventing electrons emitted from the screening sleeve to the insulator from causing flash-over via the insulator surface. In addition to the fact that discharges can then occur still between the screening sleeve and the insulator, the discharge tendency between the screening sleeve or the metal base and the earthed metal sleeve cannot be eliminated thereby.

The invention has for its object to construct a highvoltage vacuum tube of the kind set forth such that the high-voltage strength is improved without the steps to be taken in this respect resulting in a construction of substantially larger volume.

According to the invention, this problem is solved in that the end of the insulator which is connected to the metal base is provided with a circular groove which encloses a metal screen for the electrical screening of the edge of the metal base which is connected to the insulator.

The invention, which is not only used in X-ray tubes but also, for example, in neutron tubes, will be described with reference to an embodiment shown in the drawing, wherein the single FIGURE shows a sectional The cathode construction, i.e. the filament, the insulating passages 33, the cathode cap etc., are supported by a metal base 31 which, is vacuumtight connected to a cylindrical projection 21 of the ceramic insulator 2. The cylindrical projection 21 has a diameter which is slightly smaller than that of the remaining part of the ceramic insulator 2.

The vacuumtight connection of the metal base 31 to the cylindrical projection 21 can be achieved in known manner by soldering, the metal base 31 being soldered to the pre-metallised projection 21. Due to the comparatively small tensile strength of the soldered connections and because the coefficient of expansion of the metal base is generally larger than that of the ceramic material used, a so-termed external soldered joint must be made, i.e. the metal base is externally soldered to the projection 21 and at the same time encloses this projection. When the high-voltage is applied, very high field strengths would occur on the lower, comparatively sharp edge of the metal base, the said field strengths being liable to give rise to discharge phenomena in the X-ray tube. In order to avoid such discharges, a cylindrical screening sleeve 32 is provided which encloses the metal base 31 and which extends over the lower edge thereof as far as an annular, circular groove 22 which has a depth and a width of a few mm and which is provided in the end face of the ceramic insulator which is shaped as a truncated cone. The side walls of the groove extend parallel to the screening sleeve, whilst the lower end of the groove is rounded.

The groove prevents the comparatively sharp lower edge of the screening sleeve 32 from causing discharges. Comparatively high field strengths which initially lead to electron emission also occur on this edge, but the emitted electrons cannot reach the metal sleeve or the outer jacket of the insulator. These electrons will rather charge the side walls of the groove 22, a potential then being formed which decreases the field strength on the lower edge of the screening sleeve 32, thus preventing further electron emission.

So as to preclude the initial electron emission completely, the inner surface of the groove can be advantageously metallised and be connected to the metal base 31 or the screening sleeve 32 of the same potential in an electrically conductive manner.

1f the lower edge of the metal base can be rearranged downwards so far that the upper edge of the groove projects thereabove, a separate screening sleeve 32 can be dispensed with. In that case the inside of the groove 22 must definitely be metallised.

In order to increase the creepage paths, the ceramic insulator 2 can be provided with a concentric wave-like structure instead of a smooth surface. It is then advantageous to shape the wave-like structure such that the groove is situated in a thickened portion.

What is claimed is:

1. A high-voltage vacuum tube, comprising a cathode which is enclosed by a metal sleeve, the cathode being mounted on a metal base which is mechanically connected to the metal sleeve by way of an insulator, the end part of the insulator near the connection to the metal base being provided with a circular groove which encloses an end portion of a cylindrical metal screen for the electrical screening of the edge of the metal base at the connection to the insulator.

2. A high-voltage vacuum tube as claimed in claim 1, wherein the screening sleeve which encloses the metal base projects into the groove.

3. A high-voltage vacuum tube as claimed in claim 2,

wherein the inside of the groove is metallised. 

1. A high-voltage vacuum tube, comprising a cathode which is enclosed by a metal sleeve, the cathode being mounted on a metal base which is mechanically connected to the metal sleeve by way of an insulator, the end part of the insulator near the connection to the metal base being provided with a circular groove which encloses an end portion of a cylindrical metal screen for the electrical screening of the edge of the metal base at the connection to the insulator.
 2. A high-voltage vacuum tube as claimed in claim 1, wherein the screening sleeve which encloses the metal base projects into the groove.
 3. A high-voltage vacuum tube as claimed in claim 2, wherein the inside of the groove is metallised. 